The invention relates to a method of transporting oil and gas under high pressure in tanks on board a ship. The invention has been especially developed in connection with the utilization of so-called marginal fields in the North Sea. An economical utilization of the gas from finds in the North Sea requires that the gas must be recovered and transported to the consumer by means of a system which does not push the price up too high. The immediate solution that comes to mind is transportation in pipelines, but physical limitations such as the Norwegian channel, insufficiant quantity, etc. present hindrance in this regard. One should therefore have other alternatives in view, especially such as may be utilized in connection with smaller finds.
The extent to which an offshore gas and/or oil field can be utilized is among other things dependent on the economics of the recovery and transportation system one selects, and the size of the field, i.e., the quantity of the oil and gas which can be recovered, represents an important parameter. For smaller fields, the situation may be that if one is unable to find an especially simple and inexpensive system of transportation, it may not be feasible to utilize the field. Our deliberations indicate that storage and transportation of oil/gas under high pressure would represent a favourable solution. Such a solution, in brief, involves taking the entire flow from one or more oil wells on board ship in high pressure tanks and bringing it to land, where the necessary treatment, relief of pressure and separation occur. The land-based plant can serve several fields. Offshore, the field equipment can be limited to that which is necessary for connection in order to transfer the oil/gas to a tanker. However, the equipment on board the ship must be relatively advanced, especially the navigation equipment, if the system involves locating the well from the tanker. Alternatively, one can naturally utilize a loading buoy.
The invention concerns a method which is to be utilized in connection with the transportation of oil and gas under high pressure, and provides a solution to the problems one encounters in loading and unloading.
In accordance with the invention, it is proposed that loading and unloading be carried out utilizing a suitable liquid under pressure, e.g. water, in the individual tanks on board the ship, whereby during loading, a tank or a group of tanks containing pressurized liquid are filled with oil while the pressurized liquid simultaneously is displaced into the next tank or group of tanks to be filled, after which the said next tank/group of tanks are filled with cargo while the pressurized liquid becomes displaced into a third tank/group of tanks, etc, and that during unloading, the cargo from one tank or one group of tanks is removed by introducing a pressurized liquid into the tank/group of tanks, unloading of the cargo in the next tank/group of tanks occuring through the transfer of the pressurized liquid from the said first tank/group of tanks into the next, etc.
The method of the invention can be used both in connection with oil and associated gas and for gas alone. The pressure in an oil/gas well will usually (at least for a certain period of time) be much higher than 100 bar. It is assumed, however, that the most economical solution will be obtained if the pressure, by relieving the pressure through expansion of volume, can be reduced to around 100 bar.
With the invention one eliminates the drawbacks associated with pressurized filling of empty tanks, and it also permits unloading to be performed in a simple manner.
If the cargo is introduced into a non-pressurized tank, an initial pressure drop of about 100 bar over the control valve will result, and this has the following unfavourable consequences:
Firstly, much of the liquid will evaporate (flash gas) and the volume of the tank will be poorly utilized; secondly, the evaporation consumes heat, which causes a drop in temperature and freezing of the water accompanying the liquid; and thirdly, with the big drop in pressure, one will obtain high velocity flow (speed of sound) through the control valves, and sand particles accompanying the oil will thus cause much wear on the fittings and piping. The high flow speed will also produce cavitation and noise problems.
To prevent the above, one might use gas as the pressure medium, i.e., building up the gas pressure in the tanks to about 100 bar by means of compressors prior to loading, and forcing the cargo by pressure onto land by means of the same compressors. However, this would require great amounts of energy and is unfavourable from the point of view of safety as compared to pumping water. Relatively little energy is required to increase the pressure in a water-filled tank from 0 to 100 bar.
The loading and unloading rate is controlled by regulating and controlling the amount of water which is removed from or introduced into the tanks, respectively. The above-mentioned flow control problems will not be experienced, even at great drops in pressure.
During loading, the drive pressure is supplied "gratis" from the oil/gas wells, and in unloading, the drive pressure is maintained by water pumps.